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The A team on parade

Bridges Southport

An unusual construction strategy is delivering a new seaside bridge in double quick time, reports Andrew Mylius.

Blackpool's near neighbour and sometime rival resort, Southport, has gaming machines, roller coasters, candyfloss and a marine parade. But it takes some pride in being a more genteel holiday spot than its vulgar big sister - many of its residents and holidaymakers are retired.

The arrival in town of a brazenly modern newcomer could be expected to cause some pursing of lips. But in fact construction of the asymmetric cable stayed Marine Parade Bridge has ruffled not so much as a blue-rinsed feather.

Contractor Balfour Beatty and consultant Babtie have turned the £5M bridge from a competition-winning idea into a nearly completed structure in an astonishingly brief two years.

Client is Sefton Borough Council.

It is needed to span an artificial lake lying between Southport's old shoreline and a strip of reclaimed land, created in the 19th century by ambitious city engineers. A Victorian road bridge was condemned 15 years ago, leaving an ornate pedestrian bridge as the only access.

The new bridge will carry vehicles, but also provides generous pavements for the crowds that flock to Southport from April to October.

Constructed in reinforced concrete, the crossing has a slim 120m long deck straddled by a 56m tall A-frame pylon positioned a third of the way along its length. Two cable fans reach down from a lozenge-shaped steel anchorage at the pylon's head to support the main span of the deck. On the reverse of the pylon, cables are gathered together and tied back to the bridge's western abutment, which acts as a counterweight.

Conventionally, such a structure would have involved building the pylon and anchorage first, followed by incremental construction of the deck. But Balfour Beatty has been able to cram almost two and a half years' work into just over 18 months by tackling things very differently.

Work started with installation of a spread footing for the pylon and construction of a temporary 'causeway' across the shallow lake on which a falsework platform could be erected. This enabled the deck to be constructed insitu in its entirety at the same time as the pylon was being put up, says Balfour Beatty project manager Jerry Corvin.

This was important as 'it took a very complicated piece of construction off the critical path', he explains. The pylon's 2m diameter legs are inclined at 11.27infinity and densely packed with reinforcement.

'Getting the geometry right wasn't exactly easy, ' Corvin says. And there was a steep initial learning curve while the construction gang familiarised itself with the steel formwork system which was purpose designed for the job by Balfour Beatty's inhouse temporary works team.

While the pylon was going up, carpenters constructed formwork for the deck. This is relatively thin at 272mm, thanks to the use of post-tensioning in its two longitudinal beams and 22 transverse beams. With the deck extending like the growing pylon's shadow, only one operation threatened the construction schedule - installation of the anchorage.

Fabricated by local shipbuilder Balmer Lindley in 50mm steel, this 60t component was lifted into place just before Christmas and fixed with bolts and a concrete stitch pour.

Last month saw two teams from subcontractor Freyssinet scale the pylon to install the stays. These are multi-strand locked coil cables supplied by Bridon. Jacking the cables tight was carried out incrementally - 'the further you move along the deck the more horizontal stress you're putting into the pylon. By the time you stress the furthest cable stays you're pulling it back and forth by up to 60mm, ' says Babtie technical director David Slater. And cables to both sides of the deck had to be stressed simultaneously to avoid twisting, Corvin adds.

As load was fully taken up in the cable stays, the deck almost imperceptibly lifted clear of its supporting platform, allowing falsework to be struck. It is now all but removed.

Balfour Beatty must next excavate the fill used to build the causeway, returning the lakebed to its original profile. All the material is to be used for landscaping around the western abutment.

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